1. Field of the Invention
The present invention generally relates to radio receivers and receiving methods, and specifically relates to a radio receiver and receiving method for controlling the beam-width of a beam-width-variable antenna based on reception quality determined by such as carrier-to-interference ratio.
2. Description of the Related Art
In a mobile communication system such as cellular phone system, it is necessary to establish a radio entrance network connecting a plurality of base stations. One example of such radio communication system is shown in FIG. 1.
Referring to FIG. 1, each radio zone 1 is established by a base station 2 having antennas 4 with directivities 3. The directive antennas 4 establish a radio entrance network connecting base stations 2 (shown by bold arrows in FIG. 1). In this entrance network between radio stations, the antennas 4 receive not only the desired direct wave from a communicating base station, but also interference waves such as undesired waves from other base stations out of communication, or reflective waves reflected by buildings, etc. In order to improve reception quality, it is necessary to reduce the influence of interference waves, and therefore the following prior methods are known.
Referring to FIG. 2, a schematic view of circular aperture antennas is shown. These kinds of circular aperture antennas are frequently utilized in a conventional entrance network. As shown in FIG. 2, interference waves 6 in addition to a desired wave 5 come into the antennas. A beam pattern 8 or lobe shows the direction of maximum radiated power. Under condition that the interference waves 6 degrade desired wave power to interference wave power ratio or carrier-to interference power ratio (CIR), it is known to widen the antenna diameter 7 as shown in the right antenna in FIG. 2, in order to narrow the beam-width 8 of the antenna to reduce the influence of the interference waves. Among the same strength radio waves coming into the antenna from different directions, the radio wave coming along the central line of the directivity is received the most strongly, and oblique incident radio waves are received weakly, as represented by the figure of the lobe 8. In this specification, a beam-width or directivity angle means the angular separation between two directions in which radiation power is identical and is half (3 dB reduction) of the maximum power at the center. The wider the beam-width the lower the gain of the antenna is, normally.
An adaptive antenna shown in FIG. 3 is known as another technique for reducing the influence of interference waves. An adaptive antenna 9 can adaptively change its antenna beam pattern 10 in response to the reception spatial environment, to reduce the influence of interference waves. In order to improve its receiving characteristics, the adaptive antenna 9 directs the null (significantly lower gain) to the direction in which an interference wave 6 comes.
Further, a time and space equalizer is obtained by combining temporal signal processing to an adaptive array antenna. By performing temporal/spatial signal processing, it is possible to reduce the influence of a delayed wave 7 coming from the same direction as the one from which the desired wave 5 comes.
As another interference reduction technique, an interference canceler as shown in FIG. 4 is known. In the interference canceler shown in FIG. 4, a propagation path is estimated based on a received signal 44 and an estimated error of the past propagation path, and the estimated propagation path is used for generating a replica 47 for an interference wave 46. By subtracting the interference wave replica 47 from the received signal 44, carrier 48 to interference 49 power ratio (CIR) can be improved.
Among the above referenced prior interference reduction methods, the circular aperture antenna can reduce interference by enlarging its antenna diameter, but has a shortcoming in that it needs a physically wide area. The circular aperture antenna cannot meet a requirement for a broadened beam-width, especially when interference influence is insignificant and more than two communication links need to be voluntarily established for a plurality of base stations. The antenna itself has to be replaced when changing beam-widths. When making an additional line, an additional antenna has to be physically built. Further, there is another defect in that the interferences increase due to the additional lines, and therefore antennas for other lines should also be replaced.
According to the above interference reduction techniques using the adaptive array antenna, it is possible to change the directivity direction and beam-width and increase the number of lines, and therefore deal with newly added interferences. However, there are difficulties in constructing a complex system and performing increased calculating operations.
Further, the above mentioned circular aperture antenna and adaptive array antenna have physical and technical limitations regarding narrowing the beam-width thereof, and a defect that interference waves coming from the same direction as the desired wave cannot be cancelled.
According to the above mentioned interference canceller, it is theoretically possible to cancel all interference waves. However, since one additional interference wave needs one additional replica generation circuit, as the number of interference waves increases, the circuit size and calculation amount increase exponentially, resulting in difficulty of realizing the whole processing system.